In recent years, electronic appliances have been required to be compact, lightweight and high speed, and high densification of printed wiring boards has advanced. Production of a printed wiring board by a semi-additive method using electroplating has been drawing attention. Japanese Patent Application Laid-Open (JP-A) No. 10-4254 discloses a semi-additive method of making interstitial via holes (hereinafter referred to as IVHs) in a resin on which a circuit is to be formed, roughening the surface of the resin by chemical, giving a Pd catalyst thereto, conducting electroless plating to form a patterned, electroplating resist, and then forming the circuit by pattern-electroplating. This method makes it possible to form finer wiring as compared with the subtractive method that an amount of side-etching is large. Furthermore, there is a method of forming a circuit onto a resin-fitted metal foil by a semi-additive method. In order to make the thickness of a metal foil thinner, JP-A No. 2003-158364 discloses a substrate material wherein copper foils of 5 μm thickness or less are put to both faces of an insulating resin. According to this manner, failures based on short circuits in a conductor circuit are less, so that printed-wiring boards having good circuit-formability can be produced. JP-A No. 7-221444 discloses a method of forming a copper layer of about 1 μm thickness on a single face of a polyimide film by using an electron beam vapor-deposition device, and laminating the resultant onto an inner layer circuit through an adhesive or prepreg, so as to be rendered a power supply layer.
In the methods of JP-A Nos. 10-4254 and 2003-158364 out of the above-mentioned methods, the shape of the roughened surface disturbs the formation of fine wiring. Additionally, according to these methods, there is generated an inconvenience that electric properties of the resultant are lowered by the shape of the roughened surface. Since no roughened surface shape is formed in the method of JP-A No. 7-221444, the method is advantageous for forming fine wiring or the electric properties. However, the substrate itself becomes expensive. Thus, the method lacks versatility.
JP-A No. 2004-025835 discloses a method of conducting circuit-formation by using a smooth very thin copper foil as a power supply layer to form fine wiring inexpensively. In this method, IVHs are made from the upper face of the smooth very thin copper foil, which is present on an insulating layer, for interlaminar connection. Electroless copper plating is then performed, and an image of a resist is formed. Thereafter, a circuit is formed. The method for making IVHs is classified into three kinds of methods: the direct laser perforating method, the conformal perforating method, and the large window method. In the direct perforating method, a laser ray is directly irradiated onto a copper foil so as to form IVHs. In the conformal perforating method, windows having a diameter equal to that of IVHs are made by photolithography, and then a laser ray larger than each of the windows is irradiated to make the IVHs. In the large window method, windows larger than the diameter of IVHs are made in a copper foil, and then a laser ray as large as the IVH diameter is irradiated to make the IVHs. In the method described in JP-A No. 2004-025835, it is difficult to use any method other than the direct laser perforating method. This is based on the following reason: when windows are formed by photolithography, it is difficult to form a resist at the end of a substrate; therefore, the resin at the substrate end is naked. On the resin, which is smooth, electroless copper plating peel is not obtained; therefore, there are generated inconveniences such that electroless copper is peeled in the process for mass production. Alternatively, when windows larger than the diameter of IVHs are made in a copper foil by the large window method, the resin outside the IVHs is naked. In this case also, there is caused a problem that plating around the IVHs is peeled.
It has been desired to provide wiring boards which overcome the above-mentioned problems and are advantageous in the formation of fine wiring, electric properties and production costs. Additionally, it has been desired to provide wiring boards which are high in reliability and good in high frequency properties.